1. Field of the Invention
The present invention relates to spread spectrum communications systems, in particular, to spread spectrum transmitters and spread spectrum receivers which can be applied to mobile communications systems in accordance with a code division multiple access (CDMA) communications scheme for transmitting data, for example, having a transmission rate higher than that of one code channel using a plurality of codes.
2. Description of the Related Art
Reference is first made to the following documents related to the present invention:
(1) U.S. Pat. No. 5,166,951; and PA1 (2) "Application Techniques for Spread Spectrum Communications Scheme" pp. 32-33 and pp. 53-59, published by Triceps Co., on Aug. 31, 1992.
The spread spectrum communications scheme utilizes spread codes such as pseudo random codes or the like to spread transmission data over a wider bandwidth than an original data bandwidth for transmission. Since the spread spectrum communications scheme has a variety of features including high resistance to interference, utilization of this scheme in mobile communications has been rapidly increased. The CDMA communications scheme is a multiple access scheme which applies the spread spectrum communications scheme as mentioned above to a mobile communications system. A mobile communications system in accordance with the CDMA communications scheme (hereinafter referred to as the "CDMA mobile communications system") can multiplex a large number of users (channels) in the same frequency band by employing spread codes for spreading data.
Also, for transmitting data having a data transmission rate higher than a rate per code channel defined by one spread code, a scheme for assigning a plurality of spread codes to a single user (hereinafter referred to as the "multi-code transmission scheme") has been proposed. As another method of transmitting high-speed data in a CDMA mobile communications system, there has also been proposed a method of transmitting data with a single spread code by reducing a processing gain. This method, however, has a drawback that a spread gain is reduced.
The multi-code transmission scheme has capabilities of transmitting data at a high data-rate without sacrificing the features of the spread spectrum, when high data-rate transmission is required with a fixed bandwidth, by utilizing a plurality of low data-rate channels having a large spread gain, instead of reducing the spread gain.
Operations involved in a transmitter in accordance with the conventional multi-code transmission scheme will be briefly described below (see FIG. 2 in the reference (1) cited above).
In the transmitter, input data d(t) is demultiplexed by a demultiplexer into a N(an integer) number of data d1 (t), . . . , dN(t), and these N number of data d1 (t), . . . dN(t) are multiplied by corresponding message chip codes g1(t), . . . , gN(t) in corresponding spread modulators (for example, EX-OR gates), respectively, to be spread. The respective message chip code g1(t), . . . , gN(t) are synchronized with a generic chip code g0(t). Then, the spread N number of signals and the generic chip code g0(t) are added in a combiner, and subsequently transmitted from an antenna through radio modulation processing and so on.
Operations involved in a receiver in accordance with the conventional multi-code transmission scheme will be briefly described below (see FIG. 3A in the reference (1) cited above).
In the receiver, from a received signal after being subjected to radio demodulation processing and so on, the synchronization of the generic chip code g0(t) is acquired and tracked. Next, the received signal is despread using message chip codes g1(t), . . . , gN(t) synchronized with the generic chip code g0(t) to produce data dR1(t), dR2(t), . . . , dRN(t) for respective channels. Finally, the respective channel data are multiplexed to recover the received data dR(t).
As described also in the reference (2) cited above, methods of acquiring and tracking the synchronization in a receiver may be generally classified roughly into a method utilizing a correlator and a method utilizing a matched filter.
In the method utilizing a correlator, a received signal is multiplied by a spread code, and a resulting product is integrated to produce a correlation value. Then, the synchronization is acquired and tracked on the basis of this correlation value. It should be noted that a receiver shown in the reference (1) cited above employs a configuration including a correlator.
In the method utilizing a matched filter, a received signal is input to a tapped delay line, which uses spread codes as weights of taps, to produce a correlation value, and the synchronization is acquired and tracked on the basis of this correlation value. A delay interval in the tapped delay line is selected to be chip intervals of the spread codes. Since the matched filter provides an output having a large value every spread code period (see FIG. 44 in the reference (2) cited above), the matched filter may be utilized as it is for demodulation (despread) of a received signal as well as for high speed synchronization and tracking of spread codes.
In recent years, more CDMA mobile communications systems have employed the method utilizing a matched filter than the method utilizing a correlator due to the advantages of the matched filter that has high speed synchronization and tracking capabilities and can be used as it is for demodulation (despread) of a received signal.
However, if a matched filter is utilized also for demodulation in a CDMA mobile communications system employing the multi-code transmission scheme, the system requires a plurality of matched filters because of a plurality of spread codes employed therein, thereby causing a problem that a hardware scale is increased.
In addition, even if the matched filter is not utilized for demodulation, the employment of a plurality of spread codes requires a plurality of demodulation processing systems, one for each spread code, thereby inevitably increasing the size of the receiver.
It should be noted that the problem as mentioned above has been found not only in CDMA mobile communications systems but also in one-to-one individual spread spectrum communications schemes employing the multi-code transmission scheme.